Biomimetic joint actuators

ABSTRACT

In a powered actuator for supplying torque, joint equilibrium, and/or impedance to a joint, a motor is directly coupled to a low-reduction ratio transmission, e.g., a transmission having a gear ratio less than about 80 to 1. The motor has a low dissipation constant, e.g., less than about 50 W/(Nm)2. The transmission is serially connected to an elastic element that is also coupled to the joint, thereby supplying torque, joint equilibrium, and/or impedance to the joint while minimizing the power consumption and/or acoustic noise of the actuator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application to U.S. patentApplication Ser. No. 15/877,680 filed Jan. 23, 2018, which is acontinuation application to U.S. patent application Ser. No. 14/734,662,filed Jun. 9, 2015, which is a Divisional of U.S. patent applicationSer. No. 13/417,949, filed Mar. 12, 2012, which claims priority to andthe benefit of U.S. Provisional Patent Application Ser. No. 61/451,887,filed on Mar. 11, 2011, the contents of which are hereby incorporated byreference herein in their entirety.

FIELD OF THE INVENTION

This invention relates generally to powered human segmentation devices,such as lower-extremity prosthetic, orthotic, or exoskelton apparatus,and/or to humanoid robotic devices designed to emulate humanbiomechanics and to normalize function and, in particular, to devicecomponents that deliver mechanical power, and methods for controllingsuch components.

BACKGROUND

Superior biomimetic, lower-extremity augmentation systems and humanoidsystems generally modulate mechanical impedance, joint equilibrium, andtorque in accordance with gait cycle phase, walking speed, and/orterrain in a way that can emulate human behavior. In so doing, suchsystems can normalize or even augment metabolic cost-of-transport andself-selected walking speed with respect to average limb/joint functionin a typical human. Some powered prosthetic, orthotic, and exoskeletaldevices for providing and/or augmenting human joint function such thatat least a biomimetic joint response is achieved have been described inco-pending U.S. patent application Ser. No. 12/157,727 “PoweredAnkle-Foot Prosthesis” filed on Jun. 12, 2008 (Publication No.US2011/0257764 A1); co-pending U.S. patent application Ser. No.12/552,013 “Hybrid Terrain-Adaptive Lower-Extremity Systems” filed onSep. 1, 2009 (Publication No. US2010/0179668 A1); co-pending U.S. patentapplication Ser. No. 13/079,564 “Controlling Power in a Prosthesis orOrthosis Based on Predicted Walking Speed or Surrogate for Same” filedon Apr. 4, 2011; co-pending U.S. patent application Ser. No. 13/079,571“Controlling Torque in a Prosthesis or Orthosis Based on a Deflection ofSeries Elastic Element” filed on Apr. 4, 2011; co-pending U.S. patentapplication Ser. No. 13/347,443 “Powered Joint Orthosis” filed on Jan.10, 2012; co-pending U.S. patent application Ser. No. 13/356,230“Terrain Adaptive Powered Joint Orthosis” filed on Jan. 23, 2012; andco-pending U.S. Provisional Patent Application Ser. No. 61/595,453“Powered. Ankle Device” filed on Feb. 6, 2012, the disclosures of all ofwhich are hereby incorporated herein in their entireties.

In these devices the torque, the impedance, and joint equilibrium aregenerally controlled in each joint to provide at least a biomimeticresponse to a wearer of the device. Specifically, these devices mayprovide torque in advance of toe off during a gait cycle to propel thejoint. This can enable the wearer to walk faster and with less effortwhile at the same time improving gait mechanics, thereby mitigating thewearer's discomfort.

A series-elastic actuator (SEA), described in the above-referencedpatent applications, can be used to create a backdrivable jointmechanism in prosthetic, orthotic, exoskeleton, and/or humanoid devicesin which both force (torque) and impedance are controlled. Specifically,in various lower-extremity devices described in these patentapplications, the SEA typically emulates the muscle-tendon unit responsein an ankle, knee or hip device, specifically through implementation ofa positive force or velocity feedback controller that mimics acharacteristic reflex response of the joint. To this end, the SEAtypically stores energy in one phase of a gait cycle (e.g., in thecontrolled dorsiflexion phase for an ankle device) and releases thestored energy later in the gait cycle (e.g., in the powered plantarflexion phase in the ankle device). Thus, the SEA may amplify the peakpower of the actuator, thereby reducing the size and weight of the motorand the transmission. As such, devices employing an SEA may both requireless battery power and produce less acoustic noise than other roboticsystems that provide torque for propelling a joint, but that do not usean SEA. Nevertheless, the devices using an SEA (as well as those notusing an SEA) can still require substantial battery power and mayproduce noise that is unacceptable to some users in certain situations.

One of the reasons for the high power consumption and noise is thatconventional electric actuators in leg prosthetic, orthotic, andexoskeletal devices generally employ low-torque, high-speed (i.e., highrevolutions per minute (RPM) motors that are light weight but arelimited in their torque capability. For example, the EC-4Pole 30 MaxonMotor that may be employed in prosthetic and/or orthotic devices has alow-mass (about 300 grams), but has a rather modest torque capability ofabout 0.12 Newton-meter continuous tongue, and a relatively high speed(about 16,500 RPM zero-load speed). To achieve the high joint torque andlow speed required to emulate the dynamics of a biological leg jointusing a low torque, high RPM motor, a transmission having a largereduction ratio (e.g., greater than about 150:1) is generally needed.Transmissions having such high reduction ratios, when used in anactuator system to emulate the biological dynamics of ankle, knee,and/or hip joints, typically produce significant acoustic noise output.Such transmissions may also have large frictional losses and may havelow backdrivability.

A high acoustic output may draw attention to the wearer of the device,and can thus be uncomfortable or embarrassing in certain socialsituations. Moreover, high friction and poor backdrivability can resultin a relatively poor transmission efficiency, increasing the powerconsumption of the device. These two parameters can also adverselyaffect the overall control of the joint, whether for adjusting the jointposition or for applying impedance and/or force/torque. In addition,high transmission ratios can be difficult to achieve and often requiremany functional pasts, which limits system cycle life and increasesmanufacturing complexities and associated costs. Therefore, there is aneed for improved powered actuators for use in prosthetic, orthotic,exoskeleton, and/or humanoid devices.

SUMMARY

In various embodiments, the present invention provides powered actuatordevices and methods for operating/controlling such actuators so thathuman augmentation devices using these actuators can accurately modulatethe torque, joint equilibrium, and impedance applied to a human joint,while significantly reducing the power consumption and acoustic noise ofthe powered actuators. This is achieved, in part, by using a hightorque, low RPM motor that is directly coupled to a highly backdrivable,low friction, low-reduction ratio transmission, and by using an elasticelement, coupled in series with the transmission, coupling the joint towhich torque/impedance is to be supplied with the transmission.

Conventional actuators typically employ high-rpm motors and highgear-ratio transmissions using timing belts and gears, which cangenerate acoustic noise and dissipate power. In various poweredactuators described herein, an efficient, high-torque motor, such as atransverse flux motor, having low thermal dissipation is coupleddirectly to a low gear-ratio transmission, e.g., a transmission having areduction ratio of about 80:1 or less. Examples of such transmissionsinclude ball-screw and cable transmissions. These actuators are thusdirectly coupled to the robotic joint, via an elastic element coupled inseries with the low-reduction transmission, delivering high-torque withlow inertia and high efficiency to the joint. As these actuatorseliminate the belts and gears used in conventional transmissions, theycan be more durable, light-weight quiet, backdrivable, powerful,efficient, and scalable, compared to the conventional actuators.

These improved SEAs may be employed to emulate the behavior of humanmuscles and tendons. In general, low acoustic noise, force and impedancecontrollability, and high efficiency are important attributes ofbiological muscle-tendon units. An SEA including a high torque, low RPMmotor (e.g., a transverse-flux motor), a low-reduction ratiotransmission directly coupled to the motor, and tendon-like elasticelement coupled in series with the transmission can provide many ofthese attributes of biological muscle-tendon units with greater efficacycompared to traditional actuator designs currently employed in wearablerobotic systems. This particular combination of mechanical andelectromechanical elements in the improved SEAs facilitates a biomimeticactuator platform capable of emulating the natural dynamics ofbiological leg joints in tasks such as walking, stairclimbing/descending, and running, at high efficiency and controllabilitywith relatively low acoustic noise output.

Accordingly, in one aspect, embodiments of the invention feature apowered actuator for supplying one or more of an augmentation torque,joint equilibrium, and an impedance to a joint augmented by a poweredhuman augmentation device. The powered actuator includes a motor havinga dissipation constant less than about 50 W/(Nm)², and a transmissioncoupled directly to the motor. The powered actuator also includes anelastic element coupled to the joint that is also coupled, in series, tothe transmission. The powered actuator, as adapted for use in an ankle,can generate a normalized joint torque in a range from about −2.8 toabout 2.8 Nm/kg.

In some embodiments, the motor may include a high-torque motor supplyingmotor torque of at least about 0.06 Nm/kg. Alternatively, or inaddition, the motor may include a low revolutions per minute (RPM) motorhaving an RPM less than about 1500, a transverse-flux motor, or both.The actuator may be adapted to be backdrivable.

In some embodiments, the transmission has a gear ratio less than about80:1. The transmission may include a ball-screw transmission having aball nut coupled to the elastic element. The ball-screw transmission mayinclude a screw having a pitch in a range of about 2 mm up to about 10mm, which can yield the gear ratio of less than about 80:1.

In some embodiments, the elastic element may include a spring, and thepowered actuator may additionally include a cable and a joint outputpulley. In those embodiments, the cable is coupled to both the springand the joint output pulley. In some other embodiments, the transmissionincludes a ball-screw transmission having a ball nut coupled to themotor rotor and the screw is coupled to the elastic element.

In some embodiments, the motor of the powered actuator includes a motorhaving an external rotor, and the transmission includes a cable and ajoint output pulley. The cable couples the external rotor and the jointoutput pulley. The cable may be any one of a synthetic cable, a steelcable, a belt, and a chain.

The powered actuator may also include a motor encoder adapted to measureangular displacement of a rotor of the motor with respect to a stator ofthe motor, and a joint encoder adapted to measure angular displacementof the joint about a joint pivot. The motor encoder, the joint encoder,or both may include an absolute encoder. The motor encoder and/or thejoint encoder may also include a magnetic encoder having at least 13-bitresolution.

In another aspect, embodiments of the invention feature a method foraugmenting joint function using a powered human augmentation device. Themethod includes modulating one or more of joint augmentation torque,joint impedance, and joint equilibrium during a phase of a gait cycle.The modulation is achieved, in part, using a motor having a dissipationconstant less than about 50 W/(Nm)² and that is coupled directly to atransmission. The transmission is serially coupled to an elastic elementthat is coupled to the joint. The method includes energizing the motorto apply the augmentation torque to the joint during a phase of the gaitcycle, such that the applied torque normalized by weight is in a rangefrom about −2.8 Nm/kg up to about 2.8 Nm/kg. The transmission may have agear ratio less than about 80:1.

In some embodiments, the method includes energizing the motor to applystiffness (a component of the impedance) to the joint, so that energy isstored in the elastic element and power from release of the storedenergy combines with the applied motor power to achieve a positivetorque feedback response that approximates a muscle-tendon reflex. Themethod may also include energizing the motor to apply the torque toachieve a desired joint equilibrium, and subsequently shorting leads ofthe motor during a stance phase of the gait cycle to approximate amechanical clutch, such that the joint equilibrium is substantiallymaintained during a portion of the stance phase.

In some embodiments, the method includes measuring angular displacementof a rotor of the motor with respect to a stator of the motor andmeasuring angular displacement of a structure with respect to the joint.The method also include determining a state of the elastic elementbased, at least in part, on both the angular displacement of the rotorand the angular displacement of the structure. Moreover, based at leastin part on the state of the elastic element and the angular displacementon the motor, torque contribution of the motor is computed, and themodulation is adjusted, based at least in part on the computedcontribution of the motor torque.

In another aspect, embodiments of the invention feature a method forsupplying one or more of an augmentation torque, joint equilibrium, andan impedance to a joint augmented by a powered human augmentationdevice. The method includes directly coupling a motor having adissipation constant less than about 50 W/(Nm)² to a transmission. Themethod also includes coupling an elastic element to both the joint andthe transmission, whereby when the motor is energized to supply theaugmentation torque, the torque applied to the joint normalized byweight is in a range from about −2.8 Nm/kg up to about 2.8 Nm/kg.

In some embodiments, the motor may include a high-torque motor supplyingtorque of at least about 0.06 Nm/kg. Alternatively, or in addition, themotor may include a low revolutions per minute (RPM) motor having an RPMless than about 1500, a transverse-flux motor, or both. The actuator maybe adapted to be backdrivable.

In some embodiments, the transmission has a gear ratio less than about80:1. The transmission may include a ball-screw transmission having aball nut coupled to the elastic element. The ball-screw transmission mayinclude a screw having a pitch in a range of about 2 mm up to about 10mm, which can yield the gear ratio of less than about 80:1.

In some embodiments, the elastic element may include a spring, and thepowered actuator may additionally include a cable and a joint outputpulley. In those embodiments, the cable is coupled to both the springand the joint output pulley. In some other embodiments, the transmissionincludes a ball-screw transmission having a ball nut coupled to themotor rotor and the screw is coupled to the elastic element.

In some embodiments, the motor of the powered actuator includes a motorhaving an external rotor, and the transmission includes a cable and ajoint output pulley. The cable couples the external rotor and the jointoutput pulley. The cable may be any one of a synthetic cable, a steelcable, a belt, and a chain. The augmentation torque and/or the impedancemay supplied to one or more of a hip joint, a knee joint, and an anklejoint.

These and other objects, along with advantages and features of theembodiments of the present invention herein disclosed, will become moreapparent through reference to the following description, theaccompanying drawings, and the claims. Furthermore, it is to beunderstood that the features of the various embodiments described hereinare not mutually exclusive and can exist in various combinations andpermutations. As used herein, the term “substantially” means about ±10%and, in some embodiments, about ±5%.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A-1C illustrate a powered actuator employing a ball-screwtransmission, according to one embodiments for use with ankleprostheses;

FIG. 2 illustrates a powered actuator employing a cable transmission,according to one embodiment, for use with ankle prostheses;

FIGS. 3A-3C illustrate a powered actuator employing a ball-screwtransmission, according to one embodiment, for use with knee prostheses;

FIG. 4 illustrates a powered actuator employing a cable transmission,according to one embodiment, for use with knee prostheses; and

FIG. 5 is a table of certain design and operating parameters of poweredactuators according to various embodiments.

DESCRIPTION

The entire contents of each of U.S. patent application Ser. No.12/157,727 “Powered Ankle-Foot Prosthesis” filed on Jun. 12, 2008(Publication No. 2011/0257764 A1); U.S. patent application Ser. No.12/552,013 “Hybrid Terrain-Adaptive Lower-Extremity Systems” filed onSep. 1, 2009 (Publication No. US2010/0179668 A1); U.S. patentapplication Ser. No. 13/079,564 “Controlling Power in a Prosthesis orOrthosis Based on Predicted Walking Speed or Surrogate for Same” filedon Apr. 4, 2011; U.S. patent application Ser. No. 13/079,571.“Controlling Torque in a Prosthesis or Orthosis Based on a Deflection ofSeries Elastic Element” filed on Apr. 4, 2011; U.S. patent applicationSer. No. 13/347,443 “Powered Joint Orthosis” filed on Jan. 10, 2012;co-pending U.S. patent application Ser. No. 13/356,230 “Terrain AdaptivePowered Joint Orthosis” filed on Jan. 23, 2012; and co-pending U.S.Provisional Patent Application Ser. No. 61/595,453 “Powered AnkleDevice” filed on Feb. 6, 2012 are incorporated herein by reference.

In various embodiments described below, die use of an SEA in abiomimetic ankle device is described for the sake of convenience. ThatSEA mechanism can be readily adapted for use with biomimetic knee and/orhip devices also. A biomimetic ankle-foot prosthesis 100 depicted inFIGS. 1A and 1B includes an SEA 102. The SEA 102 uses a direct-drive,ball-screw based transmission system in which an electric motor 104 isdirectly coupled to a ball-screw transmission 106, which is seriallycoupled to an elastic element 108, connecting the transmission 106 to ajoint output, i.e., the ankle pivot 110. Thus, the SEA 102 appliestorque via a robotic joint, i.e., the ankle pivot 110, to an outputload, i.e., a carbon-fiber foot 112. In some embodiments, anotherelastic element may be connected between the motor 104 and the foot 112,in parallel to the serially connected elastic element 108.

The motor 104 is a high-torque, low-speed (rpm) motor, e.g., atransverse-flux motor, an “external rotor” permanent magnet motor, etc.Modern transverse-flux motors employ a high-pole-count external rotor(internal stator) and circumferentially-applied stator windings toachieve high-torque density with low winding resistance, therebymitigating many of the typical disadvantages of using other high-torquemotors in portable devices. These transverse flux motors areparticularly suited for prosthetic/orthotic/exoskeletal/humanoid devicesbecause they have a high power-to-weight ratio. Transverse flux motorsalso have lower peak-to-continuous power ratings compared to those ofother motors, thereby enabling a prosthetic/orthotic/exoskeletal deviceto operate at high power levels for longer periods without reachingthermal limits. Transverse flux motors can also provide a significantmotor dissipation reduction as defined by the motor copper loss persquare unit of torque as defined by R/k_(t) ², whereby R is the statorwinding resistance in ohms, and k_(t) is the motor torque constantmeasured in N-m/amp, thereby increasing motor efficiency. Lowerfrictional losses (generally due to the reduced number of motorrevolutions per gait cycle) in the transmission further increase theoverall efficiency of the prosthetic/orthotic/exoskeletal devices. Thedesign life of the transmission can increase also, in part due to thereduction in motor revolutions per cycle. These benefits can beleveraged by the SEAs, such as the SEA 102, using ball-screw and cabletransmissions.

Specifically, the rotor of the motor 104 is attached to a ball-screwshaft 114 using a clamping nut 116. The clamping nut applies a preloadto the axial thrust bearings 118 that serve to align the ball-screwshaft 114 radially and support the thrust imparted by the ball nut 120during actuation. The rotating screw 114 drives the ball-nut 120longitudinally which in turn drives the series spring 108 about theankle pivot beatings 110, thus providing impedance and/or torque at theankle to the foot 112. Those skilled in the art appreciate thatalternatively, in some embodiments, the rotor can be directly coupled tothe ball-nut, thereby controlling the linear translation of the screw.

Typically in an ankle device, during the controlled dorsiflexion phaseof the gait cycle, the SEA 102 delivers a programmable impedance andjoint equilibrium at the ankle joint. It should be understood that inother devices, such as hip and/or knee devices, the SEA may deliver aprogrammable impedance, joint equilibrium, and/or torque in thecontrolled dorsiflexion and/or other phases of the gait cycle. In theankle device 100, the SEA 102 thus emulates a non-linear (hardening)torsional spring impedance of the ankle pivot 110; the associated torqueis stored as potential energy in the series spring 108. The hardeningspring behavior can be accomplished through use of non-linear positiveforce or velocity feedback (as described in the various co-pendingpatent applications identified above) as a means of emulating thecalf-muscle/Achilles tendon reflex response. At or near the end of thecontrolled dorsiflexion phase, the SEA 102 applies torque and, as thefoot heel begins to lift off a surface on which the wearer is walking,the energy stored in the series spring 108 is released, like a catapult,combining with the motor applied torque to produce a positiveforce/torque feedback response to approximate a muscle-tendon reflex,thus producing at least a biomimetic response. The impedance and/ortorque applied by the motor 104 may be normalized by the wearer'sweight.

An absolute encoder may be used to measure angular displacement of themotor rotor in relation to the stator. Another absolute encoder may beused to measure angular displacement of the foot structure 112 about theankle pivot bearings 110. Instead of absolute encoders, magnetic fieldangle encoders, e.g., the RMB-20 having a 13-bit resolution,manufactured by Renishaw, may be used. The measured angulardisplacements can be used to determine the state of the motor 104, forthe purposes of commutation, torque, and/or joint equilibrium control,and of the output joint, i.e., ankle pivot 110. These motor and anklestates can be used to estimate the state of the series spring 108. (Seefor example, the co-pending U.S. patent application Ser. No. 13/079,564“Controlling Power in a Prosthesis or Orthosis Based on PredictedWalking Speed or Surrogate for Same” filed on Apr. 4, 2011; U.S. patentapplication Ser. No. 13/079,571 “Controlling Torque in a Prosthesis orOrthosis Based on a Deflection of Series Elastic Element” filed on Apr.4, 2011). In general, the motor position defines a joint equilibriumposition through simple kinematics (e.g., the law of cosines). Thedifference between that joint equilibrium position and the actual jointposition, when multiplied by a calibrated series spring constant,determines the series spring torque and, thereby, the energy stored inthe spring.

In some embodiments, based on the determined series spring state andstiffness (i.e., spring constant) of the series spring 108, force andjoint torque contribution of the SEA 102 is determined. Furthermore,based on the determined contribution of the spring force and motortorque, the torque and impedance applied by the SEA 102 and equilibriumof the joint can be modulated. (See for example, the co-pending U.S.patent application Ser. No. 13/347,443 “Powered Joint Orthosis” filed onJan. 10, 2012; co-pending U.S. patent application Ser. No. 13/356,230“Terrain Adaptive Powered Joint Orthosis” filed on Jan. 23, 2012; andco-pending U.S. Provisional Patent Application Ser. No. 61/595,453“Powered Ankle Device” filed on Feb. 6, 2012).

The SEA 102 can achieve a low gear ratio, i.e., a ratio of the motorrotor displacement and the output joint displacement that is less thanabout 30:1 or about 20:1. In one embodiment of the SEA 102, theball-Screw 114 typically delivers over about 2600 N of axial force at ascrew pitch of 12 mm, delivering over 100 Nm of torque to the footstructure 112. A dissipation constant of the motor 104 across a range ofgear ratios (e.g., from about 15:1 up to about 80:1) is less than about50 Watts/(Nm)². The motor dissipation constant is a ratio of the totalresistance R of the windings of the motor rotor and square of torqueoutput by the motor per unit current supplied to the motor, denotedk_(t) ².

In general, the torque output of a motor increases with the currentdrawn by the motor, which is related to the power supplied to the motor.However, the portion of the supplied power that is lost and dissipatedas heat is proportional to the square of the current drawn by the motor.Therefore, as more power is supplied to a motor, the fraction of thatpower that increases the torque output of the motor can be less than thefraction that is wasted in the form of heat dissipation. Therefore, themotor 104, which has a low dissipation constant, i.e.,

$\frac{R}{k_{t}^{2}}$less than about 50 W/(Nm)², can deliver high torque with low windingloss compared to other motors having a greater dissipation constant. Assuch, the motor 104 dissipates less heat, keeping the prosthesis 100cool, and also requites less power, thereby increasing battery life.

In operation, in addition to providing torque to the ankle pivot 110(e.g., at or near the end of the controlled dorsiflexion phase and/or inthe powered plantar flexion phase of the gait cycle) the motor 104 mayalso provide an impedance and joint equilibrium to the ankle pivot 110,for example, to achieve an ankle (joint, in general) equilibriumtrajectory during the swing phase of the gait cycle. Similarly, as inthe application of torque as described above, the motor 104 can causedisplacement of the ball nut 120, applying a force to the series spring108 which, in turn, provides the required impedance to the ankle pivot110 with respect to the joint equilibrium trajectory.

In some embodiments, the motor leads are shorted, such that the motordraws substantially no current and operates as a dynamic mechanicalclutch. This can enable an ankle or other augmentation device to providestability during loss of battery or system malfunction. The shortedleads mode exerts a viscous damping torque on the motor, proportional tok_(t) ²/R. As measured at the output of the transmission, the viscousdamping is amplified by the square of the gear ratio, kg, yielding atransmission damping, B, of kg²×k_(t) ²/R. For an SEA with seriesstiffness, K_(SEA), the time constant of the dynamic (viscous) clutch isB/K_(SEA). In some embodiments that store energy in the series springfor rapid release later, it is useful to apply the viscous clutch at atime when the desired spring energy is achieved. Within a small timeperiod in relation to the time constant above, the transmission iseffectively a static brake, enabling the spring to release and deliverpower to the joint. Such a mode of operation is useful in slow walking,where consistent and quiet power is desired, and in running, where theankle functions primarily as a spring, and the series spring releaseoccurs in less than 50 milliseconds. Such a mode is also useful incontrol of a knee in early stance, to deliver high torque through theseries spring with no battery power. In all of the above embodiments,the clutch is used to apply high torque but without substantiallydrawing energy from the battery.

Thus, in general, in an SEA having a certain gear ratio and a certainseries spring constant, the smaller the motor dissipation constant thelonger the duration for which the applied stiffness (a component of theimpedance) can be substantially maintained after shorting the motorleads. Thus, in an SEA using a motor having a large dissipation constantand, consequently, having a duration for which stiffness can besubstantially maintained without drawing current that is shorter thanthe time period for which the equilibrium needs to be maintained, thepower supplied to the motor cannot be turned off without adverselyaffecting the joint (e.g., ankle) equilibrium. In the SEA 102, however,if the gear ratio of the transmission 106 is about 40:1, the motordissipation constant is about 10, and the spring constant of the seriesspring 108 is about 400, the SEA 102 can maintain the applied stiffnessfor a time constant (i.e., holding time) of about 250 milliseconds.Typically during the stance phase of the gait cycle while walking orrunning, this holding time is sufficient to maintain a roughly fixedjoint equilibrium for a required duration, typically about 50-100milliseconds for walking and running. Accordingly, as the motor 104draws substantially zero current after shorting the leads, a furtherreduction in the power consumption of the SEA 102 is achieved whilesimultaneously achieving ankle equilibrium.

With reference to FIG. 1C, the three-phase stator assembly 122 of themotor 104 wraps around the rotor 124 to facilitate mounting of the motorto the prosthesis housing, e.g., using a needle bearing component. Abellows 126 protects the screw 114 from contamination. The ball-nut 120employs an end-flexure 128 to isolate the thrust bearings 118 fromout-of-plane moment loads as shown in Section A-A in FIG. 1B. Theend-flexure 128 can move side-to-side so as to eliminate side-loads,further isolating the thrust bearings 118 from moments applied by theseries spring 108. Typically, thrust loads on the end-flexure 128 aresupported by needle bearings press fit into a end-flexure mounting hole130.

FIG. 2 depicts a biomimetic ankle-foot prosthesis that uses adirect-drive rotary actuator 200 with a cable transmission. The actuator200 employs a high-torque, transverse-flux, external rotor motor 202 todirectly drive the ankle output pulley 204 via a cable 206. The cablecan be a synthetic cable or a steel cable. In some embodiments, a beltof a chain drive may be used instead of a synthetic or steel cable.Motors other than transverse flux motors, but having an external rotormay also be used. A rotary series spring connects the ankle outputpulley 204 to the ankle output joint 208. The rotor of the motor 202 maybe captivated by ankle shells using needle bearings.

Absolute angular displacement of the ankle output pulley 204 and of therotor of the motor 202 may be used, as described above with reference toFIGS. 1A and 1B to determine the state of the actuator 200. Magneticfield angle encoders may be used instead of absolute encoders. The flexin the cable 206 may be measured based on the span (length) of thecable, which is related to the difference between the output jointposition and the motor position. With high resolution encoders, cablestretch can be sensed with sufficient bandwidth and resolution forclosed-loop control. The flex in the cable 206 can then be compensatedin an output torque feedback loop. The cable 206 in the actuator 200 canachieve a gear ratio, i.e., the ratio of the motor angle and the angleof the output joint, i.e., ankle pivot 208, of about 20:1, in oneembodiment.

FIGS. 3A-3C illustrate a biomimetic knee prosthesis 300 that uses adirect-drive, ball-screw based system coupled to a series springconnecting the ball-screw transmission to the knee joint output. Thedevice 300 controls the equilibrium position of the knee joint 302, andapplies torque or impedance to the knee joint 302 substantially alongthe centerline of the output pulley 304. The knee prosthesis 300 candeliver about 200 Mm of torque over a range of about 120 degrees ofangular displacement of the artificial knee joint 302 useful for stairand steep ramp ascent as well as for level ground walking. Atransverse-flux, motor 306 (or other high-torque, external rotor motor)drives a screw 308 thereby driving a ball-nut 310 supported by a linearrail 312. A retaining nut 314 preloads the angular contact bearings 316inside the motor 306.

A cable attachment device 318, also supported by the linear rail 312,connects to the ball-nut 310 via series springs 320, 322 and linearlydrives the cable 324. The cable 324 wraps around a light-weight pulley326, an idler pulley 328, and the output pulley 304 to applytorque/impedance to the knee joint 302 about the knee axis 330. Thecable 324 can be a synthetic cable, a steel cable, a belt, or a chaindrive.

The pitch of the ball screw 308 is in the range of approximately 6 mm upto about 10 mm so as to achieve a low gear ratio of less than about30:1, of less than about 20:1. The gear ratio is a ratio of therespective angular displacements of the rotor of motor 306 and the kneejoint 302. An absolute magnetic encoder 330 having at least a 13-bitresolution (e.g. RMB-20 manufactured by Renishaw), is used to measurethe angular displacement of the motor rotor in relation to the stator,and an absolute magnetic encoder 334, which may also have at least a13-bit resolution, is used to measure the angular displacement of thelower knee structure relative to the upper knee structure. The motor andknee joint angles are used, respectively, to determine the states of themotor 306 and the knee 302, and can also be used to estimate the stateof the series springs 320, 322. For redundancy in sensing, a linearseries-spring deflection potentiometer 336 is optionally included tomeasure series-spring deflections directly. Based on the series springstate and stiffness (i.e. spring constant), series-elastic actuatorforce and joint torque supplied by the SEA can be determined. (See forexample, the co-pending U.S. patent application Ser. No. 13/079,564“Controlling Power in a Prosthesis or Orthosis Based on PredictedWalking Speed or Surrogate for Same” filed on Apr. 4, 2011; U.S. patentapplication Ser. No. 13/079,571 “Controlling Torque in a Prosthesis orOrthosis Based on a Deflection of Series Elastic Element” filed on Apr.4, 2011).

FIG. 4 depicts a biomimetic knee prosthesis that uses a direct-driverotary actuator 400 using cable transmission, similar to that describedabove with reference to FIG. 2. A high-torque external rotor motor 402(e.g., a transverse flux motor) is configured in a “pancake” arrangementto minimize stack height and to maximize torque-current gain. The motor402 drives a pulley 404 and, through a direct-cable transmission 406,drives the output pulley 408. A rotary series spring couples the kneeoutput pulley 408 to the knee output joint 410. Thus, rotating theoutput pulley 408, in turn, causes the knee joint 410 to rotate.Absolute encoders on the motor 403 and on the output pulley 408 may beused to measure the state of the actuator 400 similarly as describedabove with reference to FIGS. 1A and 1B. The SEA 400 yields a low gearratio, i.e., the ratio of the motor angle and the knee joint angle, ofabout 20:1.

Although various direct-drive SEAs are described above as components ofwearable robot ankle and knee prosthetic devices, this is forillustrative purposes only. Hip prosthetic devices are alsocontemplated. To those skilled in the art, it should be apparent thatthese SEAs can be readily adapted for use in wearable robot ankle, knee,and hip orthotic devices, wearable robots for upper-extremity orthoticand prosthetic devices, and in humanoid robots. It should also beunderstood that although the powered actuators described herein takeadvantage of some of the key attributes of transverse flux motors,specifically high torque density and efficiency, these actuators canalso leverage other high-torque motors, including hybrid steppingmotors, induction motors, traditional radially-applied permanent magnetmotors, and variable reluctance motors. The speed of these motors may beless than 5000 rpm, or less than 1500 rpm, or less than 300 rpm,depending on the optimum system design as defined by the motor,transmission gear ratio, series-spring stiffness, parallel elasticity,and battery power source; optimum generally referring to a tradeoff ofbattery economy per stride, design life of the transmission, and deviceweight. Table 1 in FIG. 5 shows various design and operating parametersof the powered SEAs according to various embodiments. The minimum,typical, and maximum values of these parameters are also listed inTable 1. A typical wearer weighs in the range from about 190 lbs up toabout 250 lbs. Wearer mass is used to normalize the actuator weight.Series Stiffness (i.e., spring constant of the serially connectedelastic element) and Parallel Stiffness (i.e., spring constant of theoptional elastic element connected in parallel with the seriallyconnected elastic element).

While the invention has been particularly shown and described withreference to specific embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes that come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

What is claimed is:
 1. A powered human augmentation device comprising: alow revolutions-per-minute (RPM) motor having an RPM less than 1500configured to apply torque to a joint of the human augmentation deviceduring a phase of a gait cycle to modulate at least one of jointaugmentation torque, joint impedance, or joint equilibrium during thephase of the gait cycle; a transmission coupled directly to the low RPMmotor; and a controller configured to short leads of the motor to exerta viscous damping torque on the motor proportional to a motordissipation constant given by R/k_(t) ², where R refers to a statorwinding resistance and k_(t) refers to a motor torque constant, thedissipation constant being less than 50 W/(Nm)².
 2. The device of claim1, wherein the motor comprises a high-torque motor supplying motortorque of at least about 0.06 Nm/kg.
 3. The device of claim 1, whereinthe device is adapted to be backdrivable.
 4. The device of claim 1,wherein the transmission has a gear ratio less than about 80:1.
 5. Thedevice of claim 1, wherein: the motor comprises an external rotor; andthe transmission comprises a cable and a joint output pulley, the cablecoupling the external rotor and the joint output pulley.
 6. The deviceof claim 1, further comprising: a motor encoder adapted to measureangular displacement of a rotor of the motor with respect to a stator ofthe motor; and a joint encoder adapted to measure angular displacementof the joint about a joint pivot.
 7. The device of claim 6, wherein atleast one of the motor encoder and the joint encoder is selected from agroup comprising: an absolute encoder, or a magnetic encoder having atleast 13-bit resolution.
 8. A method for augmenting joint function usinga powered human augmentation device, the method comprising: energizing alow revolutions-per-minute (RPM) motor having an RPM less than 1500 toapply torque to a joint of the human augmentation device during a phaseof a gait cycle to modulate at least one of joint augmentation torque,joint impedance, or joint equilibrium during the phase of the gaitcycle, the motor coupled directly to a transmission; and shorting leadsof the motor to exert a viscous damping torque on the motor proportionalto a motor dissipation constant given by R/k_(t) ², where R refers to astator winding resistance and k_(t) refers to a motor torque constant,the dissipation constant being less than 50 W/(Nm)².
 9. The method ofclaim 8, wherein the viscous damping torque is exerted on the motorduring a loss of battery power.
 10. The method of claim 8, wherein thedissipation constant is about 10 W/(Nm)2.
 11. The method of claim 8,further comprising energizing the motor to apply stiffness to the joint,the elastic element configured to store energy and to release the storedenergy as power, the motor applying power to augment the power of theelastic element to achieve a positive torque feedback response.
 12. Themethod of claim 8, further comprising: energizing the motor to apply thetorque to achieve a desired joint equilibrium position; and shortingleads of the motor during a stance phase of the gait cycle tosubstantially maintain the joint equilibrium position during a portionof the stance phase.
 13. The method of claim 8, further comprising:measuring an angular displacement of a rotor of the motor with respectto a stator of the motor; measuring an angular displacement of astructure with respect to the joint; determining, using a hardwarecontroller, a state of the elastic element based, at least in part, onboth the angular displacement of the rotor and the angular displacementof the structure; computing, based at least in part on the state of theelastic element and the angular displacement of the motor, a torquecontribution of the motor using the hardware controller; and adjustingthe modulating, based at least in part on the computed contribution ofthe motor torque.
 14. A powered human augmentation device comprising: alow revolutions-per-minute (RPM) motor having an RPM less than 1500configured to apply torque to a joint of the human augmentation deviceduring a phase of a gait cycle to modulate at least one of jointaugmentation torque, joint impedance, or joint equilibrium during thephase of the gait cycle; a transmission coupled directly to the low RPMmotor; an elastic element serially coupled to the transmission, theelastic element coupled to the joint; and a controller configured toshort leads of the motor to exert a viscous damping torque on the motor.15. The device of claim 14, wherein the elastic element comprises aspring, the device further comprising a cable and a joint output pulley,the cable being coupled to both the spring and the joint output pulley.16. The device of claim 14, wherein the transmission comprises aball-screw transmission having a ball nut coupled to the motor rotor andthe screw coupled to the elastic element.
 17. The device of claim 14,wherein the motor comprises a high-torque motor supplying motor torqueof at least about 0.06 Nm/kg.
 18. The device of claim 14, wherein thedevice is adapted to be backdrivable.
 19. The device of claim 14,wherein the transmission has a gear ratio less than about 80:1.
 20. Thedevice of claim 14, further comprising: a motor encoder adapted tomeasure angular displacement of a rotor of the motor with respect to astator of the motor; and a joint encoder adapted to measure angulardisplacement of the joint about a joint pivot.